GENERAL CHARACTERS 231 
relate to the prehension of food generally, are essentially adaptive 
and consequently plastic or prone to variation, and hence can- 
not well be relied upon as tests of affinity. In another character, 
also adaptive, the laxity of the connection of the ribs with the 
vertebral column and with the sternum, and the reduction of that 
bone in size, allowing great freedom of expansion of the thoracic 
cavity for prolonged immersion beneath the water, the Mystacocetes 
have passed beyond the Odontocetes in specialisation. On the other 
hand, the greater symmetry of the skull, the more anterior position 
of the external nostrils and their double external orifice, the form 
of the nasal bones, the presence of a distinctly developed olfactory 
organ, the mode of attachment of the periotic bone to the cranium, 
the presence of a cecum and the regular arrangement of the 
alimentary canal, the more normal characters of the manus and the 
better development of the muscles attached to it, and the presence, 
in many species at least, of parts representing not only the bones 
but also the ligaments and muscles of a hind limb,! all show less 
deviation from the ordinary mammalian type than is presented by 
the Odontocetes. Taking all these characters into consideration, it 
does not appear reasonable to suppose that either type has been 
derived from the other, at all events in the form in which we see 
it now, but rather that they are parallel groups, both modified in 
different fashions from common ancestors. 
Among the Mystacocetes, in the especially distinguishing 
characters of the division, the Rorquals are less specialised than the 
Right Whales, which in the greater size of the head, the length and 
compression of the rostrum, the development of the baleen, and 
shortness of the cervical region, are exaggerated forms of the type, 
and yet they retain more fully some primitive characters, as the 
better development of the hind limb, the pentadactylous manus, 
and the absence of a dorsal fin. Both types are found distinct in 
a fossil state at least as far back as the early Pliocene age, but: 
generally represented by smaller species than those now existing. 
Some of the Pliocene Rorquals (Cetotherium) were, in the elongated 
flattened form of the nasal bones, the greater distance between the 
occipital and frontal bone at the top of the head, and the greater 
length of the cervical vertebrae, more generalised than those now 
existing. In the shape of the mandible also, Van Beneden, to 
whose researches we are much indebted for a knowledge of these 
forms, discerns some approximation to the Odontocetes. 
Among the last-named group there are several distinct types, of 
which that represented by Platanista, although in some respects 
singularly modified, has been considered to present on the whole 
approximations towards the more normal and general type of 
1 These have been described in detail by Professor Struthers in the Journal of 
Anatomy and Physiology, 1881. 
